Move DM png code to picture_utils, for use by other tools.

tools/picture_utils.h

Index: tools/picture_utils.h
diff --git a/tools/picture_utils.h b/tools/picture_utils.h
index 1373020a5b294db7851edd02fb91c9cf928bbeac..49a2c821a46087974505b3470bcd7d5898f201ec 100644
--- a/tools/picture_utils.h
+++ b/tools/picture_utils.h
@@ -8,7 +8,9 @@
 #ifndef picture_utils_DEFINED
 #define picture_utils_DEFINED
 
-class SkBitmap;
+#include "SkBitmap.h"
+
+class SkData;
 class SkString;
 
 namespace sk_tools {
@@ -47,6 +49,12 @@ namespace sk_tools {
     bool write_bitmap_to_disk(const SkBitmap& bm, const SkString& dirPath,
                               const char *subdirOrNull, const SkString& baseName);
 
+    // Return raw unpremultiplied RGBA bytes, suitable for storing in a PNG. The output
+    // colors are assumed to be sRGB values. This is only guaranteed to work for the
+    // cases that are currently emitted by tools:
+    // Linear premul 8888, sRGB premul 8888, Linear premul F16
+    sk_sp<SkData> encode_bitmap_for_png(SkBitmap bitmap);
+
 } // namespace sk_tools
 
 #endif  // picture_utils_DEFINED